Certain epoxy compositions have been suggested as photosensitive resists. For example, the use of certain reaction products of a monoethylenically unsaturated acid and certain epoxides for the manufacture of patterned articles such as printed electronic circuits has been suggested in U.S. Pat. No. 3,661,576 to Crary and abandoned U.S. patent application Ser. No. 613,784 entitled "Photo-Processable Coatings" to Schmidt et al and Assigned to the Assignee of the Instant Application. Furthermore, photosensitive composition based on epoxide resins which have not been pre-reacted with a monoethylenically unsaturated acid but which must include certain specific types of sensitizers have been suggested for use in preparing selective or patterned coatings. An example of such prior art is U.S. Pat. No. 3,930,856 to DeMoira.
Also, various coating compositions containing monoethylenically unsaturated acid-epoxy adducts have been suggested for coating operations, which do not involve the problems of resolution experienced in the use of a composition as a photosensitive resist in the manufacture of patterned articles such as printed circuits. Exemplary of such disclosures are U.S. Pat. No. 3,912,670 to Huemmer et al; U.S. Pat. No. 3,772,062 to Shur et al; U.S. Pat. No. 3,450,613 to Steinberg et al and U.S. Pat. No. 4,003,877 to Lipson, et al.
Providing coating compositions which exhibit acceptable imaging (resolution and acuity) characteristics to be suitable as an image or pattern mask particularly for integrated circuits along with the necessary physical characteristics to provide a permanent protective coating is quite difficult. In particular, compositions suitable as a patterned protective coating for a circuit board must possess a number of different types of properties. In many instances, according to the specific use and/or environment of use, and depending upon priorities, a balancing of properties becomes necessary. In particular, a satisfactory level of one property is achieved by a reduction in the performance of another property. In many coatings, for instance film forming properties are sacrificed in order to obtain satisfactory resolution and acuity, and likewise, acuity has been sacrificed, when possible, to achieve desirable film forming characteristics.
In the manufacture of printed circuit boards, it is desirable that the polymeric material used be patternable by means of a photolytic process, as is the practice with photoresist, and which also, unlike photoresist, can be retained to perform one or more additional functional uses during or after fabrication of the circuit board. In view of the unique combination of good properties obtainable by the products of the present invention, such functions, for example, use as a permanently retained etch mask, solder mask, plating mask, dielectric film or protective coating are possible.
According to the present invention, it has been found that a particular combination of reaction products of a monoethylenically unsaturated carboxylic acid with two different epoxides in combination with the other constituents of the composition provide photo-processable coatings which not only exhibit good resolution (e.g.--sharply defined acuity), but also possess excellent film forming properties such as good flexibility and toughness. Moreover, it has been noted that the loss of film of the cured portion during the development when practicing the present invention is much less than when using other photoprocessable epoxy coatings including those disclosed in abandoned U.S. patent application Ser. No. 613,784 referred to hereinabove.
A further advantage of the present invention, is that the compositions can be made non-tacky. Accordingly, such can be applied in a process which employs the following steps:
(a) Coating the material onto a carrier film such as an inexpensive polyethylene carrier;
(b) Laminating the film and carrier to a circuit board;
(c) Removing the carrier; and
(d) Then exposing and developing the coating composition.
On the other hand, the commercially available photoresist compositions which are applied utilizing a carrier in accordance with the process shown in the examples of U.S. Pat. No. 3,469,982 to Celeste are not generally suitable for such a process. For instance, the commercially available photoresists employ an expensive transparent carrier (e.g.--Mylar) and the photoresist is exposed through the carrier. The general commercially available photoresists are somewhat tacky and accordingly the carrier cannot be separated from it prior to the exposure since portions would stick to the exposure mask. If the carrier were removed portions, would be removed along with the carrier.
An advantage of being able to successfully remove the carrier before the exposure step is that exposing through the carrier as shown in the examples of U.S. Pat. No. 3,469,982 causes some degradation of the image.
It is believed that generally the commercially available photoresists include a relatively large amount of liquid cross-linking agent; whereas, the compositions of the present invention can be prepared with only relatively minor amounts of a polyethylenically cross-linking agent since the majority of cross-linking can occur through the chain molecules themselves.